Content

Semaphores can be described as counters which are used to provide synchronization between processes or between threads within a process for shared resources like shared memories. System V semaphores support semaphore sets where each one is a counting semaphore. So when an application requests semaphores, the kernel releases them in sets. The number of semaphores per set can be defined through the kernel parameter SEMMSL.

To see all semaphore settings, run:

# ipcs -ls

The SEMMSL Parameter

This parameter defines the maximum number of semaphores per semaphore set.

Oracle recommends SEMMSL to be at least 250 for 9i R2 and 10g R1/R2 databases except for 9i R2 on x86 platforms where the minimum value is lower. Since these recommendations are minimum settings, it’s best to set it always to at least 250 for 9i and 10g databases on x86 and x86-64 platforms.

NOTE:
If a database gets thousands of concurrent connections where the ora.init parameter PROCESSES is very large, then SEMMSL should be larger as well. Note what Metalink Note:187405.1 and Note:184821.1 have to say regarding SEMMSL: “The SEMMSL setting should be 10 plus the largest PROCESSES parameter of any Oracle database on the system”. Even though these notes talk about 9i databases this SEMMSL rule also applies to 10g databases. I’ve seen low SEMMSL settings to be an issue for 10g RAC databases where Oracle recommended to increase SEMMSL and to calculate it according to the rule mentioned in these notes. An example for setting semaphores for higher PROCESSES settings can be found at Example for Semaphore Settings.

The SEMMNI Parameter

This parameter defines the maximum number of semaphore sets for the entire Linux system.

Oracle recommends SEMMNI to be at least 128 for 9i R2 and 10g R1/R2 databases except for 9i R2 on x86 platforms where the minimum value is lower. Since these recommendations are minimum settings, it’s best to set it always to at least 128 for 9i and 10g databases on x86 and x86-64 platforms.

The SEMMNS Parameter

This parameter defines the total number of semaphores (not semaphore sets) for the entire Linux system. A semaphore set can have more than one semaphore, and as the semget(2) man page explains, values greater than SEMMSL * SEMMNI makes it irrelevant. The maximum number of semaphores that can be allocated on a Linux system will be the lesser of: SEMMNS or (SEMMSL * SEMMNI).

Oracle recommends SEMMNS to be at least 32000 for 9i R2 and 10g R1/R2 databases except for 9i R2 on x86 platforms where the minimum value is lower. Setting SEMMNS to 32000 ensures that SEMMSL * SEMMNI (250*128=32000) semaphores can be be used. Therefore it’s recommended to set SEMMNS to at least 32000 for 9i and 10g databases on x86 and x86-64 platforms.

The SEMOPM Parameter

This parameter defines the maximum number of semaphore operations that can be performed per semop(2) system call (semaphore call). The semop(2) function provides the ability to do operations for multiple semaphores with one semop(2) system call. Since a semaphore set can have the maximum number of SEMMSL semaphores per semaphore set, it is often recommended to set SEMOPM equal to SEMMSL.

Oracle recommends to set SEMOPM to a minimum value of 100 for 9i R2 and 10g R1/R2 databases on x86 and x86-64 platforms.

Setting Semaphore Parameters

To determine the values of the four described semaphore parameters, run:

# cat /proc/sys/kernel/sem
250 32000 32 128

These values represent SEMMSL, SEMMNS, SEMOPM, and SEMMNI.

Alternatively, you can run:

# ipcs -ls

All four described semaphore parameters can be changed in the proc file system without reboot:

# echo 250 32000 100 128 > /proc/sys/kernel/sem

Alternatively, you can use sysctl(8) to change it:

# sysctl -w kernel.sem=”250 32000 100 128″

To make the change permanent, add or change the following line in the file /etc/sysctl.conf. This file is used during the boot process.

# echo “kernel.sem=250 32000 100 128” >> /etc/sysctl.conf

Example for Semaphore Settings

On systems where the ora.init parameter PROCESSES is very large, the semaphore settings need to be adjusted accordingly.

As shown at The SEMMSL Parameter the SEMMSL setting should be 10 plus the largest PROCESSES parameter of any Oracle database on the system. So if you have one database instance running on a system where PROCESSES is set to 5000, then SEMMSL should be set to 5010.

As shown at The SEMMNS Parameter the maximum number of semaphores that can be allocated on a Linux system will be the lesser of: SEMMNS or (SEMMSL * SEMMNI). Since SEMMNI can stay at 128, we need to increase SEMMNS to 641280 (5010*128).

As shown at The SEMOPM Parameter a semaphore set can have the maximum number of SEMMSL semaphores per semaphore set and it is recommended to set SEMOPM equal to SEMMSL. Since SEMMSL is set to 5010 the SEMOPM parameter should be set to 5010 as well.

Hence, if the ora.init parameter PROCESSES is set to 5000, then the semaphore settings should be as follows: